The present invention relates to a system for controlling an inverter to generate a predetermined power output during certain external processes. More particularly, the present invention provides a control system for an inverter delivering power to a backlight display that reduces electromagnetic interference during read/write commands. Particular utility for the present invention is in Palm Computer devices, or other computer devices where the LCD panel and the system board are in relative close proximity to one another, although the present invention has equal utility in any application where it is desirable to control the output of the inverter during certain computer system processes.
Palm Computers typically comprise a pen which interfaces between the users and the CPU via an LCD panel. A sensor detects pressure from the tip of the pen and sends the appropriate commands to the CPU. A D/A converter is provided that receives the analog signal generated by the sensor and converts this signal to a digital signal to be executed by the CPU. The A to D converter interfaces between the pen and the CPU. Since the size of a typical Palm Computer is relatively small, the mother board is mounted in close proximity to the LCD panel. LCD panels include a control module, a DC to AC inverter, and one or more cold cathode fluorescent lamps. During normal operation, the LCD panel radiates electromagnetic waves that can interfere with the read/write process of the CPU, or other components on the mother board of the Palm Computer. The A/D converter is particularly susceptible to electromagnetic interference.
One solution to alleviate the electromagnetic interference between the LCD display and the components of the motherboard is shielding. However, shielding an LCD panel has proven to be expensive, not reliable, and not very effective. Also, shielding adds undesirable weight to small computer systems such as Palm Computers.
FIG. 1 depicts a lamp driving circuit 10 that includes a CCFL controller for driving one or more cold cathode fluorescent lamps 14 via a transformer 16 and a resonant tank circuit that includes a secondary side of the transformer 16 and capacitor 18. A feedback circuit 20 is provided to provide a DC signal indicative of lamp current conditions, and is utilized by the CCFL controller to adjust power to the load. The CCFL controller can include inverter topologies well known in the art, for example, full bridge, half bridge or push-pull inverter topologies. The solution to electromagnetic interference depicted in FIG. 1 includes an on/off signal, generated externally, that shuts off the switching mechanisms (i.e., half bridge, full bridge or push pull switches) on command. The command to shut off the CCFL controller via on/off signal 22 may be generated by the system microprocessor (not shown) during periods where electromagnetic energy coming from the lamp would interfere with read/write processes of the microprocessor, memory, or the D/A converter interfacing between the pen and the microprocessor.
Disadvantageously, by shutting off the controller 12, even for small periods of time, no drive signals are supplied to the switches, and hence, the CCFL controller generates zero volts to the transformer (and the load). As is well understood in the art, CCFL lamps require a high voltage striking period to initially strike the lamp, followed by steady state period where lower lamp voltage can be supplied to operate the lamp. Typically, the striking voltage is on the order of 1500 volts and steady state voltage is on the order of 500 to 600 volts. In the solution depicted in FIG. 1, every time the controller 12 receives a command via on/off signal 22 to turn on the lamp, the controller 12 must go through the striking period to first strike the lamp. CCFL controller may include a soft xe2x80x9csoft startxe2x80x9d or frequency sweeping functionality to provide lamp strike, and in any event, require several hundred milliseconds to strike the lamp. Thus, if the microprocessor shuts off the controller 12 to perform a read/write process which may take only 2 or three milliseconds, when the controller 12 is turned on again the lamp needs to be struck, so the whole process may take several hundred milliseconds to complete. This approach may introduce a noticeable flicker on the LCD display.
In one exemplary embodiment, the present invention provides a lamp load control system. The system includes a lamp controller comprising an inverter to generating an AC signal from a DC signal, a load coupled to the inverter, and a feedback circuit coupled to the load generating a feedback signal indicative of power supplied to the load. The system also includes a command signal generator generating a command signal indicative of a preferred power output of said inverter. The command signal is combined with the feedback signal to cause the controller to temporarily reduce power delivered to the load.
In another exemplary embodiment, the present invention provides another lamp load control system. The system includes a lamp controller comprising an inverter generating an AC signal from a DC signal, a load coupled to the inverter, and a feedback circuit coupled to the load generating a feedback signal indicative of power supplied to the load. The system also includes a command signal generator generating a command signal indicative of a preferred power output of said inverter. The controller receives the feedback signal and the command signal and temporarily reduces power delivered to the transformer based on the value of said feedback signal or said command signal.
The present invention further provides a method to control a lamp load. The method comprises the steps of: supplying power to a lamp; generating a feedback signal indicative of power supplied to the lamp; generating a command signal indicative of a preferred power delivered to the lamp; combining the feedback signal and the command signal; and temporarily reducing the power delivered to the lamp.
The present invention provides yet another method to control a lamp load, comprising the steps of: supplying power to a lamp; generating a feedback signal indicative of power supplied to the lamp; generating a command signal indicative of a preferred power delivered to the lamp; and temporarily reducing power delivered to the lamp based on the command signal or the feedback signal.
It will be appreciated by those skilled in the art that although the following detailed description will proceed with reference being made to exemplary embodiments, the present invention is not intended to be limited to these exemplary embodiments. Other features and advantages of the present invention will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and wherein: